Field of the Invention
The present invention relates to a work vehicle, in particular, a hybrid-type work vehicle, and to a method for controlling the same.
Background Information
Recently, a hybrid-type work vehicle has been proposed that travels using driving power from an engine and driving power from a motor. A hydraulic-mechanical transmission (HMT) or an electric-mechanical transmission (EMT) is disclosed as a power transmission device for hybrid-type work vehicles as in, for example, Japanese Unexamined Patent Application Publication No. 2006-329244.
The HMT has a planetary gear mechanism, and a first pump/motor and a second pump/motor connected to rotating elements of the planetary gear mechanism. The first pump/motor and the second pump/motor function as either hydraulic motors or hydraulic pumps in response to the travel state of the work vehicle. The HMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the pump/motors. The HMT has a third pump/motor. The third pump/motor is provided to assist either the first pump/motor or the second pump/motor. The pump/motors assisted by the third pump/motor are switched in response to the speed range of the vehicle speed.
An electric motor is used in the EMT in place of the hydraulic motor in the HMT. That is, the EMT has a first generator/motor, a second generator/motor, and a third generator/motor. The first and second generator/motors function as either electric motors or electric generators in response to the travel state of the work vehicle. The third generator/motor is provided to assist either the first generator/motor or the second generator/motor in the same way as the above-mentioned third pump/motor. Similar to the HMT, the EMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the generator/motors.